Think about it game-theoretically:
If you live as if you'll die, and you don't, you'll have a much more interesting life, because you took advantage of it thinking you're living under the Sword of Damocles.
If you live as if life extension will keep you going, and it fails, you'll have a short, boring life, full of things you put off until it was too late.
Those are the extreme values of the matrix; if you work through the rest, I think you'll find that the most utility is always gained by living as if you'll die eventually.
(Living as if you'll die tomorrow is romantic and all, but it would prevent you from working towards any goal which takes more than less than a day to complete. It could also land you in prison, deported to a less-pleasant foreign country, or both.)